The present invention relates to molding processes and apparatus and to blow molded articles. Specifically, the present invention relates to blow molding processes and apparatus which improve the venting of trapped air from molded material and which improve the surface finish on molded articles, such as automobile fascias, bumpers or other body components.
Fundamentally, blow molding is a manufacturing technique that involves the use of compressed gas to expand material, usually a thermoplastic, outward against the inner walls of a surrounding, split-cavity mold. In extrusion blow molding methods and apparatus, molding is accomplished by first extruding a preselected length of material, for example, heat-softened thermoplastic, into a tubular preform, also called a parison. The parison is then surrounded, and usually pinched at one or more locations, by the split-cavity mold. A blow pin is then inserted into the parison and compressed gas is injected through the blow pin to inflate the parison, much like a balloon, against the sides of the mold. Typically, low gas pressures are used. Contact of the thermoplastic material with the mold causes the material to cool and harden in the desired shape. The split-cavity mold is then separated and the molded article is removed.
In the manufacture of blow molded articles such as automobile body components, it is often desirable to provide a very smooth, aesthetically pleasing surface finish. However, it is difficult to obtain smooth, high quality surface finishes on products molded using conventional blow molding techniques without further surface finishing, such as sanding and painting. This difficulty arises, in part, from the presence of air or other gas at or near the surface of the molded article. Trapped air or gas may cause variations or defects in the surface finish of the molded product. Blow molding is especially susceptible to the undesireable effects of trapped air because of the use of compressed gases in the molding process and because of the displacement of air from the mold due to the expansion of the parison.
Conventional blow molding techniques have addressed this problem by providing vent holes at spaced intervals on the mold surface to facilitate venting of trapped air. However, vent holes provide for venting of trapped air only in local regions on the surface of the molded product; some trapped air may not reach vent holes during the molding process. To provide more uniform venting, it is known to utilize porous metal molds. However, porous molds are prone to clogging after repeated use and require additional effort to maintain. Thus, there remains a need for blow molding processes and apparatus which provide uniform and sufficient venting of trapped air and high quality surface finishes, yet which do not require significant maintenance effort.
The aforementioned problems are eliminated and desired advantages are realized by the present invention, which contemplates new and improved processes and apparatus for blow molding by providing mold surfaces with micrograined patterns to improve venting and surface finish. The micrograined patterns are preferably in the form of a pattern of projections which define between them a network of interconnected channels. The projections are of a sufficient height that they engage the outer surface of the fully-expanded parison. The channels are of a sufficient depth that, when the parison engages the projections, the channels form venting passages with the parison surface to permit trapped air to escape and migrate through vent holes which communicate with the channels. The dimensions of the projections are selected to provide for the formation of venting passages while preventing the formation of patterns in the surface of the molded product.
In one aspect, the invention may be defined as an apparatus for molding articles, the apparatus comprising a mold having a mold surface provided with a micrograined pattern which facilitates the formation of venting passages with the surface of an expanded parison. More particularly, the invention may be defined as an apparatus for blow molding articles in which the mold surface is provided with a micrograined pattern that includes projections of a substantially circular or parallelogram shape, the projections defining a network of interconnected channels which form venting passages with the surface of the expanded parison.
In another aspect, the invention may be defined as a process for blow molding articles, the process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison forms venting passages with the micrograined pattern. More particularly, the invention may be defined as a process for blow molding articles, the process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern, the pattern including projections of a substantially circular or parallelogram shape defining a network of interconnected channels; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison engages the projections and forms venting passages with the channels.
In yet another aspect, the invention may be defined as a blow-molded article, formed by a process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison forms venting passages with the micrograined pattern. More particularly, the invention may be defined as a blow-molded article, formed by a process comprising the steps of: a) providing a parison of material to be expanded outward into a mold; b) providing a mold having a mold surface including a micrograined pattern, the pattern including projections of a substantially circular or parallelogram shape defining a network of interconnected channels; c) providing a parison of material within the mold; d) expanding the parison outward against the mold surface such that an outer surface of the parison engages the projections and forms venting passages with the channels.